Blender Sine wave calculated incorrectly?
There is nothing wrong, your expectation and the parameters of the modifier just do not fit together. I guess this is caused by a unit mash-up of radians, degrees and frames.

I'll try to sum up the necessary math as short as possible to avoid further misunderstanding. Take a look at the XY plot with a circle given a radius of 1 (known as unit-cirle) and consider the marked triangle:



The sine is now the Y position of the black dot (green line) and depends on the position of the dot running counterclockwise around our circle. We can either measure the marked angle in degree or radians. Plotting this with reference to the angle measured in radians gives us this plot:



You can observe, that a full period is reached after 2 PI, which is approx. 6,28. Now compare this to a sine modifier applied to the x-location of some object:



Looks similar, right? Both plots are bounded by +1 and -1 and complete a full period after 6,28. But in the Blender case 6,28 is the number of FRAMES, after which the sine starts all over again. 

Now it depends on your use case if you want such a fast oscillation. You can stretch and shift the sine with the four given parameters as you like. The equation seems to be this one (c=constans):



Hope this helps!